The operations of wireless communications systems in the United States have traditionally been regulated by the Federal Communications Commission (FCC) to reduce the likelihood of electromagnetic interference between wireless communications systems operating within the same frequency range and close geographic locations. Historically, the FCC has addressed the electromagnetic interference issue by licensing wireless communications system operators and assigning such licensed operators the usage of specific radio frequencies to be utilized in a particular geographic location. Because the operations of these wireless communications systems were limited to a relatively small number of operators, "clear channel" frequencies could be assigned by the FCC to wireless communications operators on a limited geographic basis. Typically, the same frequency assigned to a wireless communications system operator was not reused by another wireless communications system operator within a 65 to 150 mile radius, depending on the system parameters such as antenna height, transmitter power, and service type. In an electromagnetic environment where there are few licensed operators, geographic separation between operators is large, and wireless communications system transmitter movement is infrequent, the FCC coordination method for interference control is quite adequate.
Advances in wireless communications system technology have permitted the introduction of wireless communications systems for the mass market place, and allowed the extensive use of cellular radio telephones and radio paging systems for operation in both business office and home environments. The rapid growth of wireless communications operations in these environments is radically impacting the historical coordination method for reducing electromagnetic interference between wireless communications systems. For example, cellular radio telephones and radio paging systems have become increasingly popular for mobile communications involving everyday business and home situations. Thus, the wireless communications signal environment for any one geographic location may vary over a period of time as mobile wireless communications systems are moved by operators into and away from a specific site. Also, as the number of wireless communications systems operators increases in a geographic location, signal density and environment complexity also increases, and thereby further enhances the likelihood of electromagnetic interference in a particular business or home environment. Therefore, advances in wireless communications system technology have provided an electromagnetic environment where there are a relatively large number of operators, geographic separation is small, and transmitter movement is frequent. Any regulatory attempt to use traditional licensing and record-keeping methods to reduce electromagnetic interference between these wireless communications systems will not be adequate, particularly in view of the labor-intensive activity required to achieve such wireless communications system operation coordination.
Also, the historical coordination method for reducing electromagnetic interference between wireless communications systems is not utilized by the FCC to control the operation of small, very low power wireless communication devices that are licensed under the Code of Federal Regulations, 47 C.F.R. 15. Specifically, the operations of FCC Part 15 regulated devices are not coordinated by specific frequency assignment or fixed geographic locations. These FCC Part 15 regulated devices include wireless local area networks, cordless telephones, wireless business telephone systems, and automatic door openers. Because of the advances in wireless communications technology, these devices are also commonly found in both business office and home environments. One can also expect the signal density and complexity provided by these FCC Part 15 regulated devices to continue to increase as the popularity of these devices increases in the marketplace.
Therefore, the reduction of electromagnetic interference, in a dense and complex environment provided by mass-produced, and often-times mobile, wireless communications systems, is an issue that requires an innovative solution to overcome the inherent difficulties of traditional regulation methods. This electromagnetic interference problem is particularly highlighted when many different wireless communications systems are operated without coordination and within a close proximity to each other, such as the operation of these devices inside an office building.
Focusing upon this office building example, the wireless communications signal environment currently found in a typical business office is complex and dense, and thus is susceptible to electromagnetic interference. These electromagnetic interference signals may be provided either by radiating sources within the office itself, or by sources outside, but within close proximity to, the office. Portable and hand-held cellular radio telephones, radio paging systems, cordless telephones, wireless business telephone systems, and wireless local area networks may all be utilized for wireless communications within a business office or offices located in a particular office building. Thus, physical separation between such wireless communications systems is no longer easily attained within this business office environment. Also, the limited availability of frequencies within the electromagnetic spectrum prohibits each potential user within a business office environment from utilizing a "clear channel" frequency for interference-free wireless communications. Thus, electromagnetic interference is likely to occur between several wireless communications systems operating within the same indoor environment. Furthermore, the interference signal environment in a typical business office is subject to change; as office tenants upgrade or vary their wireless communications systems installations, or as office tenants move their wireless communications systems from offices or buildings, the electromagnetic signal environment changes. In addition to potential interference problems, the reflections of wireless communications signals from the interior surfaces of an office environment will also create signal amplitude, signal phase, and tune-delay spread problems that disrupt communications within the indoor environment. Therefore, the requirement exists for a system to flexibly control the electromagnetic environment within an indoor location, such as a business office, or a building containing many business offices.
Previous approaches for controlling electromagnetic signal propagation within an enclosure have provided complete shielding of the enclosure either to trap the broad electromagnetic signal environment within the enclosure, or to prevent the spectrum of electromagnetic signals from entering the enclosure. Typically, enclosure shielding methods have involved the use of highly-conductive metal sheets, foil, or wire mesh surrounding a structure to prevent electromagnetic interference within the enclosure. This complete shielding approach addresses the issue of electromagnetic interference, but does not permit selective frequency, amplitude, or phase control of the indoor electromagnetic environment.
In U.S. Pat. No. 2,793,245 to Dunn, a radio-wave shielded enclosure is provided to completely shield the interior of the enclosure from interference by stray radio waves. The radio-wave shielded enclosure is made of separate screening panels fabricated from radio-wave shielding materials; the panels are modified to be assembled and bolted together at a desired location.
In U.S. Pat. Nos. 4,740,654, 3,322,879, 3,070,646, and 2,860,176, all to Lindgren, double-isolated electrically shielded screen rooms are provided of various constructions. The double-screen shields are fabricated from a variety of materials known to possess shielding characteristics that prevent electromagnetic and electrostatic wave penetrations of the enclosure. The double-screen shield construction provides two complete screen shields, each wholly encompassing the included space and electrically isolated from each other, that maintain complete electrical conduction continuity between each shield.
In U.S. Pat. No. 4,806,703 to Sims, a modular system, including a number of shielding panels each having a support or frame covered with a layer of conductive material, is provided to isolate sensitive equipment from broad spectrum interference because of ambient electromagnetic radiation. A shielding layer overlies a substrate at the face of the panel and extends around the panel edges to maximize conductivity between adjacent panel edges.
The prior art also recognizes that certain materials display electromagnetic characteristics useful for controlling the electromagnetic signal environment to reduce signal interference. Certain materials are known to reduce electromagnetic interference by either cancellation or absorption of electromagnetic signals. Materials are also available to provide a reflective surface that redirects incident electromagnetic signals away from the surface.
In U.S. Pat. No. 4,514,586 to Waggoner, a shielding material is provided comprising a nonconductive base material combined with a metal layer of electrolessly-deposited copper overlain with a second layer. The shielding material typically covers the enclosure of an electronic device and is utilized to protect such equipment from electromagnetic interference or to prevent the unintended transmission of electromagnetic interference signals by these electronic devices.
In U.S. Pat. No. 4,865,834 to Tanihara et al., platelet-like magnetite and maghemite particles, useful as electromagnetic wave absorbing and shielding material, are provided for shielding devices that are the sources of electromagnetic waves.
In U.S. Pat. No. 4,867,795 to Ostertag et al., platelet-like pigments based on iron oxide are provided for electromagnetic screening of electronic devices.
In U.S. Pat. No. 4,869,970, to Gulla et al., a shielding coating is provided for attenuating electromagnetic radiation interference emitting from electronic equipment.
In addition, the prior art recognizes several methods for absorbing electromagnetic signals by the use of selected materials. Carbon fibers, carbonyl iron power, or ferrite can be interspersed within a selected material to obtain a specified absorptive characteristic in a selected frequency band. Alternatively, for broadband absorbers, multiple tapered or graded absorption layers comprising carbon filings or carbon powder are utilized to obtain absorptive characteristics over a broad frequency range.
A radio wave cancellation technique recognized in the prior art provides radio wave cancellation by positioning a highly-conductive metal sheet approximately one-quarter wavelength distance from a 377 ohms per square resistive sheet. Radio waves passing first through the front surface of this assembly, the resistive sheet, are cancelled by this technique. Alternatively, positioning the highly-conductive metal sheet between 377 ohms per square resistive sheets, each placed one-quarter wave length distance on either side of the metal sheet, produces an assembly that provides radio wave cancellation characteristics for both front and rear assembly surfaces.
The prior art also recognizes that reflective surfaces are easily constructed from highly-conductive wire mesh or highly conductive metal sheets. This construction method is commonly referred to as sheathing. Good electrical bonding between adjacent conductive sheets covering a large surface is assured by extending tabs from the edges of the sheet material such that adjacent tabs maintain contact between adjacent sheets.
Frequency selective surfaces exhibiting band-pass, band-reject, high-pass, and low-pass characteristics are also known in the prior art. For example, an evenly spaced grid of wire appears as a reflective surface so long as the spacing is small compared to a signal wavelength. By controlling the spacing of the wire grid, the frequency selective surface can provide a high-pass filter characteristic for passing electromagnetic signals above a selected cut-off frequency. By further example, the prior art also provides a technique for forming a band-pass filter by periodically performing a highly conductive sheet with frequency sensitive element geometries, including rectangular slot, circular slot, annular slot, four-legged symmetrically loaded slot, and three-legged loaded slot. Other frequency sensitive element geometries can be periodically spaced as an array on a conductive sheet to create low-pass, high-pass, band-pass, or band-reject filters. Examples of such geometrics include: dipoles, crossed dipoles, dual periodic strips, and Jerusalem crosses.
Although shielded enclosures and materials or techniques for providing selected electromagnetic characteristics are individually recognized in the prior art, the prior art does not provide a comprehensive system for optimally and flexibly controlling an indoor electromagnetic signal environment to establish and maintain a selected set of desirable electromagnetic characteristics within the enclosure. There is a need for an indoor electromagnetic signal propagation control system to provide interference rejection by preventing electromagnetic interference signals from entering an enclosure to interfere with an enclosed communications system or by preventing signals generated by the communications system within the enclosure from exiting and either interfering with another nearby exterior communications system or being intercepted by another exterior communications system. Also, there is a need to customize the indoor electromagnetic signal environment, both to provide communications with interior areas otherwise blocked from signal propagation and to absorb signal reflections from interior surfaces that may cause interference for the enclosed communications system. Furthermore, there is a need for a frequency selective shield to selectively reject or accept electromagnetic signals passing into and exiting from the enclosure to permit desired communications between a second communications system inside the enclosure and another communications system outside the enclosure or to prevent undesired interception of signals radiated from the communications system within the enclosure. Also, there is a need for an indoor electromagnetic signal propagation control system constructed with aesthetically appealing building materials manufactured to include conductive, absorptive, reflective, or frequency selective electromagnetic characteristics. In addition, there is a requirement for an indoor electromagnetic signal propagation control system that provides a flexible and easy installation method to permit modification of the indoor electromagnetic environment as required to match interference signal environment variations occurring inside or outside an enclosure over a period of time.